1. Field of the Invention
This invention relates to a semiconductor optical phase modulator suitable for use in integrated electro-optical semiconductor devices. More particularly, the invention relates to a semiconductor optical modulator comprising a sequence of semiconductor layers having a refractive index profile which minimizes optical propagation loss through the waveguide without sacrificing modulator efficiency.
2. Description of the Prior Art
Semiconductor optical modulators have been described in the art previously. However, such prior art devices generally must trade-off between reducing optical propagation loss through the waveguide and increasing or maintaining modulator efficiency. That is, these prior art designs suffer either increased loss from optical absorption by modulator electrodes when increased efficiency is sought or reduced efficiency generally caused by increased separation of electrodes from the region of optical confinement when lower loss devices are sought. Also, the prior art devices were extremely limited in the optical mode size in single mode devices. Such mode size is critical for coupling single mode devices to optical fibers and the greater the mode size permitted the easier and more efficient the coupling to such optical fibers.
Typical optical propagation losses in prior art integrated optic waveguide devices on semiconductor substrates have been in the order of several dB/cm, which is unacceptable for many device applications. While recent advances in semiconductor integrated optics have resulted in reduced waveguide propagation losses (below 0.3 dB/cm), this work has not been extended to guided wave modulators where optical absorption by modulator electrodes can increase the loss above that of a simple waveguide. In these prior art devices electrode absorption can increase the loss from the 0.3 dB/cm level by as much as 5 dB/cm in certain devices, e.g., Schottky barrier devices. Also losses reported for p-i-n modulators have been 2 dB/cm or more and the reported efficiencies (.eta.) have been between 2-3 degrees/V-mm. Although it has been shown that reduced losses are achievable by reducing the separation between the guided optical mode and the electrodes, this approach reduces the device efficiency (e.g., phase shift per applied voltage per device length).
Details of the aforementioned prior art may be found in the following publications which are incorporated herein by reference: K. Hiruma, et al, Appl. Phys. Lett., 47,186 (1985); E. Kapon, et al, Appl. Phys. Lett., 50,1628 (1987); R. J. Deri, et al, Appl. Phys. Lett., 51,791 (1987); P. Buchmann, et al, Electron. Lett., 20 (7), 297 (1984); S. H. Lin, et al, Electron. Lett., 22,935 (1986); and R. G. Walker, et al, Electron. Lett., 19,950 (1983).